1. Field of the Invention
This invention relates to improvements in the art of frying vegetables, particularly potato chips, and especially to a hot oil pretreatment of potato slices prior to frying which lowers the percentage of fat in the fried product, improves solids recovery, conserves energy, and prevents sticking of the potato slices during cooking.
2. Prior Art
Potatoes are tubers cultivated primarily for the value of the starch granules present in the cells. Potatoes usually contain less than 20% solids, and common cooking procedures such as baking and frying use thermal or microwave energy to drive off a portion of the included water. Fried potato products are particularly favored as convenience snack foods and potato chips, for example, are available in most grocery stores and many restaurants. Potato chips are prepared in automated plants using a number of processing machines, many of which have been patented (by way of illustration, see U.S. Pat. Nos. 2,091,675 and 2,179,035) and by performing a number of common steps which have been the subject of frequent modification (see U.S. Pat. Nos. 4,283,437 and 2,611,705). Processes to make fried potato products directly from potatoes involve some combination of these basic steps: (1) peeling, (2) slicing, (3) washing, (4) frying in edible oils, (5) seasoning. Slices are usually washed after slicing and before frying to remove surface starch which otherwise causes the chips to stick to each other during frying. Starch build-up in the fryer is a related problem in high-volume operations.
The washing process is done in any of a variety of devices which use warm water and agitation to separate and wash the slices (see U.S. Pat. Nos. 3,223,024, 4,251,895 and 4,272,554 for exemplary processes). Coincidentally, the designs usually provide a means whereby slices too small to be included in the commercial product are selected out, and the starch and fragments are removed as the process water is changed. Many processes also use water to cool and lubricate the slicer and/or to convey slices from the slicer to the wash tank. The wash water may be filtered to remove the starch and recycled. The wash water must eventually be discharged and replaced and must usually be treated to meet discharge permit specifications (effluent pollution control requirements).
Wash water adheres to the surfaces of the washed potato slices. If the potato slices are fried immediately after washing, this surface water may constitute a significant amount (e.g., about 15%) of the total water removed by evaporation during frying. Added water in a hot oil fryer also contributes to loss of oil due to the "steam distillation" of the oil in a two-phase system.
An associated problem encountered in frying sliced potatoes and other starchy vegetables is clumping during frying. Even after much of the surface starch has been washed off, the soft surface of the slice is still somewhat sticky and slices tend to adhere to each other in the cooking oil, producing lumps or products which are unevenly cooked and which must be discarded. Numerous attempts have been made to circumvent this problem. Vigerstrom (U.S. Pat. No. 3,997,678) described a process for heating the potato by means of an electric current passed through the wash water. Cardis et al (U.S. Pat. No. 4,251,895) described a combination of air jet and suction to remove water, starch and adherent potato fragments between washing and frying steps, and also described a modified fryer to remove eddy currents in the cooking oil (U.S. Pat. No. 4,392,420).
Other attempts to produce a satisfactory product have avoided frying. Mancuso et al. (U.S. Pat. No. 3,402,059) soaked the potato slices in hot oil and then baked them in an oven. Vogt (U.S. Pat. No. 4,068,572) described a convection oven and rack for keeping potato products separate during cooking. Wicklund et al. (U.S. Pat. No. 4,277,510) described a process in which the potatoes are boiled in water before slicing, then washed and baked in an oven. Yuan et al. (U.S. Pat. No. 4,283,425) described a process for coating the washed slices with albumin and cooking in a microwave oven.
Another undesired effect of the traditional methods for frying foods is the high percentage of lipid which remains in the final product. Typical lipids (fats and oils) used in deep frying are triglycerides which are readily saponified and absorbed in the small intestine. Triglycerides provide twice the calories per gram of carbohydrates and proteins, and typical potato chips contain as much as 40% lipid by weight. Many of the alternative processes described for making foods which are similar in appearance to fried foods yield a product lower in lipid, but achieve the result by inclusion of more process steps, the addition of more ingredients, or by less efficient heat-transfer steps. For example, Murray (U.S. Pat. No. Re. 27,531) describes a process for recoating a blanched potato with amylose starch to provide a barrier to oil uptake. The result of these processes rarely has the uniquely satisfying taste, texture or appearance of a good potato chip, french fry, or similar food product.